Pollutants
Fig.1 Schematic drawing, causes and effects of air pollution: (1) greenhouse effect, (2) particulate contamination, (3) increased UV radiation, (4) acid rain, (5) increased ground level ozone concentration, (6) increased levels of nitrogen oxides.
Major primary pollutants produced by human activity
include:
·
Sulfur oxides (SOx) - especially sulfur dioxide, a chemical compound
with the formula SO2. SO2 is produced by volcanoes
and in various industrial processes. Since coal and petroleum often contain
sulfur compounds, their combustion generates sulfur dioxide. Further oxidation
of SO2, usually in the presence of a catalyst such as NO2,
forms H2SO4, and thus acid rain.This is one of the causes for concern over the
environmental impact of the use of these fuels as power sources.
·
Nitrogen oxides (NOx) - especially nitrogen dioxide are expelled from high temperature
combustion, and are also produced naturally during thunderstorms by electric discharge. Can be
seen as the brown haze dome above or plume downwind of
cities. Nitrogen dioxide is the chemical compound with the formula NO2.
It is one of the several nitrogen oxides. This reddish-brown toxic gas has a
characteristic sharp, biting odor. NO2 is one of the most
prominent air pollutants.
·
Carbon monoxide (CO)- is a colourless, odourless, non-irritating but very
poisonous gas. It is a product byincomplete combustion of
fuel such as natural gas, coal or wood. Vehicular exhaust is a major source of
carbon monoxide.
·
Volatile organic compounds - VOCs are an important outdoor air
pollutant. In this field they are often divided into the separate categories of
methane (CH4) and non-methane (NMVOCs). Methane is an extremely
efficient greenhouse gas which contributes to enhanced global warming. Other hydrocarbon VOCs are also significant
greenhouse gases via their role in creating ozone and in prolonging the life of
methane in the atmosphere, although the effect varies depending on local air
quality. Within the NMVOCs, the aromatic compounds benzene, toluene and xylene
are suspected carcinogens and may lead to leukemia through prolonged exposure.
1,3-butadiene is another dangerous compound which is often associated with
industrial uses.
·
Particulates,
alternatively referred to as particulate matter (PM), atmospheric particulate
matter, or fine particles, are tiny particles of solid or liquid suspended in a
gas. In contrast, aerosol refers to particles and the gas together. Sources of
particulates can be manmade or natural. Some particulates occur naturally,
originating from volcanoes, dust storms, forest and grassland fires, living
vegetation, and sea spray. Human activities, such as the burning of fossil
fuels in vehicles, power plants and various industrial processes also generate
significant amounts of aerosols. Averaged over the globe, anthropogenic
aerosols—those made by human activities—currently account for about 10 percent
of the total amount of aerosols in our atmosphere. Increased levels of fine
particles in the air are linked to health hazards such as heart disease, altered
lung function and lung cancer.
·
Persistent free radicals connected
to airborne fine particles could cause cardiopulmonary disease.
·
Toxic metals, such as lead and mercury, especially their compounds.
·
Chlorofluorocarbons (CFCs)
- harmful to the ozone layer emitted from products currently banned from
use.
·
Ammonia (NH3) - emitted from agricultural processes.
Ammonia is a compound with the formula NH3. It is normally
encountered as a gas with a characteristic pungent odor. Ammonia contributes
significantly to the nutritional needs of terrestrial organisms by serving as a
precursor to foodstuffs and fertilizers. Ammonia, either directly or
indirectly, is also a building block for the synthesis of many pharmaceuticals.
Although in wide use, ammonia is both caustic and hazardous.
·
Odors — such as from garbage, sewage, and industrial processes
·
Radioactive pollutants - produced by nuclear explosions,
nuclear events, war explosives, and natural processes such as
the radioactive decay of radon.pollutants
include:
·
Particulates
created from gaseous primary pollutants and compounds in photochemical
smog. Smog is a kind of air pollution; the word
"smog" is a portmanteau of smoke and fog. Classic smog results from
large amounts of coal burning in an area caused by a mixture of smoke and
sulfur dioxide. Modern smog does not usually come from coal but from vehicular
and industrial emissions that are acted on in the atmosphere by ultraviolet light from the sun to form secondary
pollutants that also combine with the primary emissions to form photochemical
smog.
·
Ground level ozone (O3) formed from NOx and VOCs.
Ozone (O3) is a key constituent of the troposphere. It is also an
important constituent of certain regions of the stratosphere commonly known as the
Ozone layer. Photochemical and chemical reactions involving it drive many of
the chemical processes that occur in the atmosphere by day and by night. At
abnormally high concentrations brought about by human activities (largely the
combustion of fossil fuel), it is a pollutant, and a constituent of smog.
·
Peroxyacetyl nitrate(PAN) - similarly formed from NOx and
VOCs.
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